From original repository:

     Verified Compliance: Passes all 9k+ tests in the official html5lib-tests suite (used by browser vendors).

Yes, browsers do you use it. But they handle a lot of stuff differently.

    selectolax  68%  No  Very Fast  CSS selectors C-based (Lexbor). Very fast but less compliant.

The original author compares selectolax to html5lib-tests, but the reality is that when you compare selectolax to Chrome output, you get 90%+.

One of the tests:

  INPUT: <svg><foreignObject></foreignObject><title></svg>foo

It fails for selectolax:

  Expected:
  | <html>
  |   <head>
  |   <body>
  |     <svg svg>
  |       <svg foreignObject>
  |       <svg title>
  |     "foo"
  Actual:
  | <html>
  |   <head>
  |   <body>
  |     <svg>
  |       <foreignObject>
  |       <title>
  |     "foo"

But you get this in Chrome and selectolax:

    <html><head></head><body><svg><foreignObject></foreignObject><title></title></svg>foo
    </body></html>